Memory is the recollection of past experiences. Almost everyone forgets something occasionally and, typically, forgetfulness increases as a person grows older. Normal aging also may result in trouble learning new material or requiring longer time to recall learned material. Age-associated memory impairments are currently estimated to affect at least 16% of everyone over the age of 50 (Tully et al., Nature Rev. Drug Discov., 2:267-277, 2003). Mild memory loss, while a nuisance, does not usually affect a person's normal day-to-day functioning. Other forms of memory loss can be more severe and have a functional impact. Common causes of memory loss of various severities include, among others, aging, Alzheimer's disease, neurodegenerative illness, head trauma or injury, seizures, general anesthetics (such as halothane, isoflurane, and fentanyl), alcoholism, stroke or transient ischemic attack (TIA), transient global amnesia, drugs (such as barbiturates or benzodiazepines), electroconvulsive therapy (especially if prolonged), temporal lobe brain surgery, brain masses (caused by tumors or infection), herpes encephalitis or other brain infections, and/or depression.
Long term potentiation (LTP) is considered to be the cellular basis of learning and memory and is dependent on synaptic plasticity (Bliss and Collingridge, Nature, 361:31-39, 1993), which can be defined as the long-lasting strengthening of the connections between two nerve cells. Synaptic plasticity in turn is believed to be dependent on a complex interplay of protein kinases, phosphatases, and transcription factors that ultimately give rise to long-term changes in the connections between nerve cells (Gaiarsa et al., Trends Neurosci. 25:564-570, 2002). Put more simply, a particular experience is registered in the brain as a circuit-specific pattern of neural activity and, due to synaptic plasticity, the structure of the circuit is modified so as to form a memory.
LTP was originally discovered in the hippocampus but has since been observed in other regions of the brain including the cerebral cortex, cerebellum and amygdala (Malenka and Bear, Neuron, 44:5-21, 2004). One model of learning postulates that the hippocampus is the gateway to long-term memory and, once the hippocampus has registered a memory, the memory is propagated to relevant portions of the cortex for storage (e.g., visual memory to visual cortex, auditory memory to auditory cortex, etc.) (for reviews, see, Tully et al., Nature Rev. Drug Discov., 2:267-277, 2003; Adams and Sweatt, Annu. Rev. Pharmacol. Toxicol., 42:135-163, 2002).
Neurotrophic factors, which modulate short- and long-term changes in neurons of the central nervous system (CNS), have been suggested to play roles in neuronal plasticity such as learning and memory (Lo, Neuron, 15:979-981, 1995; Thoenen, Science, 270:593-598, 1995). Neurotrophic factors support the survival, differentiation and functional maintenance of nerve cells. Because of these properties, neurotrophic factors have the potential to treat a variety of chronic and acute disorders of the CNS, including memory loss. However, many classical neurotrophic factors, such as nerve growth factor, are not well suited for therapeutic purposes due to their large size and proteinaceous nature (Levy et al., BioDrugs, 19:97-127, 2005). Thus, the identification of small molecules that can mimic some or all of the properties of neurotrophic factors could have great potential for treating CNS disorders, such as memory deficits.
Twenty eight different flavonoids, including representatives of all of the six different flavonoid classes (e.g., flavanones, flavan-3-ols, flavonols, flavones, anthocyanidins and isoflavones; U.S. Department of Agriculture, USDA database for the flavonoid content of selected foods, Beltsville, Md.:U.S. Department of Agriculture; 2003), were previously assayed for their ability to promote neurite outgrowth in PC12 cells (Sagara et al., J. Neurochem., 90:1144-1155, 2004). Among the flavonoids tested, only four were found to promote PC12 cell differentiation and of these, fisetin (3,7,3′,4′,-tetrahydroxyflanone; a flavonol) was by far the most effective. Fisetin has an EC50 for differentiation of 5 μM and at 10 μM routinely induces the differentiation of 75-80% of the cells The other three flavonoids that induced differentiation of PC12 cells, Iuteolin (5,7,3′,4′-tetrahydroxyflavone; a flavone), quercetin (3,5,7,3′,4′-pentahydroxyflavone; a flavonol) and isorhamnetin (3′-methoxy-3,5,7,4′-tetrahydroxyflavone; a flavonol), had EC50s of 10 μM and at best induced the differentiation of only 50% of the cells.
The induction of differentiation by fisetin was dependent on the activation of the Ras-ERK cascade because inhibitors of this cascade blocked differentiation (Sagara et al., J. Neurochem., 90:1144-1155, 2004). In addition to promoting nerve cell differentiation fisetin has also been shown to protect nerve cells from oxidative stress-induced death (Ishige et al., Free Radic. Biol. Med., 30:433-446, 2001). However, the structural features of fisetin that underlie its functions of promoting nerve cell differentiation and/or protecting such cells from oxidative damage are unknown. Also unknown is whether the ability of fisetin to promote nerve cell differentiation and/or protect neural cells from oxidative damage have in vivo correlates.
A need exists for the identification of small molecules that can mimic some or all of the properties of neurotrophic factors. Such molecules have potential for treating CNS disorders, such as memory loss.